Babies born before 37 completed weeks of pregnancy are called premature. While many of these babies do well, some go on to have lifelong health problems. Approximately 60,000 infants with birth weights under 1500 g (about 1.5% of all newborns) are born in the United States each year and about 20% of these infants develop chronic lung disease (Births: final data for 2003. Hyattsville, Md.: National Center for Health Statistics, Centers for Disease Control and Prevention, 2005).
Severely premature infants have underdeveloped lungs and insufficient surfactant to maintain stable lung volumes. This condition may lead to Respiratory Distress Syndrome (also called hyaline membrane disease) and progress to chronic lung disease, a major contributor to preterm infant morbidity and mortality. Chronic Lung disease in the premature is associated with infants requiring mechanical ventilation via endotracheal tube.
Continuous positive airway pressure (CPAP) has been a mainstay for the treatment of preterm infants in respiratory distress for many years. CPAP provides maintenance of the mean airway pressure throughout the breath cycle to help open and maintain unstable alveoli, which are typically underdeveloped and surfactant deficient. CPAP is frequently applied to patients using commercially available mechanical ventilators. CPAP can be applied to infants either nasally or via an endotracheal tube. Unfortunately, the vast majority of mechanical ventilators are not designed to be used with nasal prongs.
The purchase and maintenance costs of mechanical ventilators render them impractical for use as CPAP devices. CPAP via endotracheal tube is known to increase chronic lung disease in preterm infants. Bronchopulmonary dysplasia makes up the majority of infants with chronic lung disease. Mechanical ventilator devices are complex to operate, requiring a substantial investment to acquire and maintain the devices, as well as, to train the care givers to properly administer the treatment. Importantly, mechanical ventilator devices do not provide broadband oscillations in airway pressure. The high frequency oscillatory ventilator systems described in U.S. Pat. Nos. 4,805,612 4,481,944 and 5,752,506, for example, are capable of delivering large oscillations in airway pressure. However, these devices can only deliver large oscillations in airway pressure at a single frequency (selected by the operator).
Conventional Bubble CPAP (B-CPAP) is thought to improve ventilation in premature infants. By bubbling mechanical ventilator CPAP gases through a fluid with a simple conduit that is submersed vertically in the fluid, B-CPAP causes an infant's chest to vibrate at high frequencies such that the infant breathes at a lower respiratory rate than an infant receiving simple ventilator CPAP. Pillow, et al. (6 (Pediatr Res 57: 826-830, 2005)) demonstrated in a mechanical lung model that B-CPAP creates oscillations in airway pressure with predominant frequencies in the ranges of about 10 to 20 Hz and 40 to 100 Hz, for example. In another study, Pillow et al. (7, Am J Respir Crit Care Med Vol 176. pp 63-69, 2007) showed that B-CPAP applied to preterm lambs breathing spontaneously had improved oxygen levels and tended to reach stable lung volumes at lower airway pressures than lambs receiving CPAP generated by a mechanical ventilator. The B-CPAP device used in the studies by Pillow et al., is described in U.S. Pat. No. 6,805,120 entitled “Breathing Assistance Apparatus.” Pillow et al. attributed the improved lung stability to the broadband frequency spectrum of oscillations in airway pressure produced by CPAP gas bubbles exiting the vertically oriented conduit submersed in water. The device described by Pillow et al., however, produces small amplitude pressure oscillations that are delivered at a relatively high range of frequencies to the airway of the host, resulting in low amplitude and low time duration pressure waves that do not deliver sufficient gas to the host's lungs.
Nekvasil, et. al., (1992 {hacek over (C)}s. Pediat., 47, 8:465-470) demonstrated that high frequency oscillations in airway pressure can be created using a B-CPAP device comprising a glass funnel placed horizontally under a fluid. Placed in this configuration the device provides higher amplitude oscillations in airway pressure, but at a narrow frequency band with low time duration. Thus, although the amplitude of oscillations is high for one frequency (about 1.1 Hz), the volume of gas delivered to the patient is still inadequate because the time duration of the pressure wave is not long enough to push sufficient amounts of gas into the patient's lungs.
Presently, in many neonatal intensive care units, preterm infants requiring respiratory assistance are place on nasal B-CPAP. If the infants fail to meet established gas exchange criteria, they are intubated and placed on mechanical ventilation. A device is needed that can maintain gas exchange and alveolar stability in infants failing B-CPAP and that reduces the number of infants requiring intubation and mechanical ventilation. A respiratory assistance device is needed that can reduce the work of breathing of patients and stabilize the lungs by maintaining mean airway pressures throughout the breath cycle. It is also desirable to provide better gas exchange than that provided by single frequency ventilators, B-CPAP, or funnel B-CPAP devices. For infants requiring mechanical ventilation, a device is need that can be applied via either nasal prongs or endotracheal tube at low peak airway pressures. Additionally, it is desirable to provide a respiratory assistance device that is simple in design, easy to operate, and inexpensive to manufacture.